1. Field of the Invention
The present invention relates to highquality digital printing in which objects are intermixedly printed, and more particularly, in which a printing process exhibits characteristic, statistically predictable defects which degrade quality and whose characteristics vary sufficiently slowly over time. For systems which pre-compensate for such predictable defects by modifying the digital data in advance, it is possible to measure known sample prints at appropriate intervals and use the results of such measurement to modify the pre-compensation functions so as to more closely track variations in each defect's signature.
2. Description of the Related Art
Digital color printers form a digital image for each of several separations, such as cyan, magenta, yellow, and black. The digital image instructs the printing mechanism in the amount of each color ink to deposit and the method of deposition at each addressable point on the page.
A digitally imaged page can consist of graphical objects such as text, lines, fills, pictures, etc, all imaged in ways which can be isolated from each other, can abut one another at one or more points, can partially overlap, one another, or can completely overlap one another. The resulting printed page or graphic image is therefore made up of a patchwork of shapes representing the graphic objects, some of which are "clipped" by objects imaged later in the succession.
In practice, every color printing system has characteristic defects which can cause subtle problems that detract from achieving the highest possible quality color printing. For example, ink jet printing must handle excessive ink coverage which can cause bleeding or spreading of colors and paper distortion. Xerographic printing contends with a different set of problems which can detract from print quality. Examples are "haloing", in which toner in one separation interferes with toner transfer at the same location in another separation, "tenting", which is toner deletion caused by high toner pile casting a mechanical or electrostatic "shadow" which prevents correct development of abutting toner, trail-edge deletion and starvation, which cause toner deletion at certain edges, or misregistration between two colors. Many of these characteristic problems in printing systems can be traced to undesirable interactions between abutting colors on the page.
Despite known problems, the digital image sent to the printer has in the past assumed a perfect printing mechanism, and provided an ideal image to print. While increasingly sophisticated controls have been added to printing mechanisms to reduce defects and come closer to the perfect printer expected by the digital image, electro-mechanical defects in any printing system are still common and are to be expected at both the low end where system cost restraints preclude use of expensive controls and the high-end where production speeds challenge existing control systems.
Recent work has begun to look at modifying a digital image in advance in order to pre-compensate for expected problems in a printer. The work may be divided into three groupings. A first grouping of prior art does "object-based compensation", which predicts and pre-compensates for printing problems unique to each object type (text, fill, image, etc.). A second grouping of art does "tapping compensation", which predicts and pre-compensates for only one printing problem: misregistration between two abutting colors. A third grouping of art does a more generalized, object-optimized "edge-defect compensation."
The first (object-based) grouping deals with individual printing objects such as text, fill, or picture, without reference to other adjacent objects. Different object types have different predictable printing problems. For example, a large uniform color fill can contain visible mottle in what should be smooth color, because the random noise of the print mechanism causes tiny variations in the amount of color put on the page; a "quiet" halftone that masks engine noise can be used in printing such a fin. Text can show fuzzy edges if the normal halftone resolution is too large; a small halftone can be chosen to print text with sharper edges. Graphics can have dull colors while images can have unreal colors; the solution is to pre-compensate by choosing a different color transform for graphics than for images.
The second (trapping) grouping of prior art is more limited in scope in that it attempts only to pre-compensate for a single painter defect caused by adjacent colors: misregistration. If a printer misregisters between separations, an thin unwanted white or color line occurs when certain adjoining colors don't abut perfectly. This second group of inventions doesn't care about individual object types or a large range of printer defects as the first (object-based) grouping does. Instead, this group of prior art simply looks at the edge between two color areas, attempting to predict when two abutting colors could cause a thin line problem if the printing system misregisters. The solution used is to generate a fixed-width, constant color fill (a "frame" or "trap") whose color and position is calculated with various methods from the two abutting colors, and to superimpose that new digital signal with the original signal to produce prints that show the misregistration problem less.
The third (generalized edge-defects) approach combines both object and color information to predict and correct a wider range of adjacency problems in a novel way. Unlike the group one (object-based) inventions above which use object information to predict individual object printing problems, the third approach uses object information to help predict and solve printing problems caused by object adjacencies. Unlike group two (trapping) inventions above which only correct for misregistration, the third approach significantly extends the range of adjacency problems that can be detected and corrected. Detection of a larger number of adjacency problems is made possible by including not just color information in predicting adjacency problems but also object information such as object type, object size in the scan and process directions, rendering intent, and other relevant object parameters. Pre-compensation/correction of a larger number of adjacency printing problems (beyond simply misregistration) is made possible by using a novel approach different from the simple trapping solution of adding a uniform-width, constant-color frame between two adjacent colors. Instead, a function is applied to an object edge that can change both its color and rendering hint anamorphically (that is, differently in the process or scan directions) as a function of the distance from the edge.
All three of the approaches to digital data pre-compensation are described in the patent application Rumph et al., application Ser. No. 09/222,486 filed Dec. 28, 1998, titled "Anamorphic Object Optimized Function Application for Printer Defect Pre-Compensation", which is incorporated herein by reference.
The focus of this invention is not on the method used to pre-compensate digital data to reduce printer defects, but rather on achieving long term stability in the results. If the extent and severity (the "signature") of a particular defect changes over time, it will not be optimally effective to apply a fixed pre-compensation to the defect based on one or a few measurements made at only one particular point in time.
It is well known in the art to use color measurements to maintain color fidelity in color printers. Printer characterization is done to map requested colors to c,m,y, k values used by the actual printer. Then, printer calibration is done to prevent the printer from drifting in its color representations. To do this, color patches with known values are printed at periodic intervals. The actual printed color values are measured, and by comparing the actual measurements with the expected values, color transform data can be modified to correct for the current state of the printer.
U.S. Pat. No. 5,416,613 to Rolleston et. al. describes a method of calibrating a printer using a test image comprising a number of randomly located test patches, some of which may be repeated to constrain printer non-uniform color response at different points on the page.
U.S. Pat. No. 5,537,516 to Sherman et. al. presents a method for calibrating a printer by using various measurement devices and generating a set of calibration curves as modifications of each of the individual color print channels (e.g., CMYK).
U.S. Pat. No. 5,739,927 to Balasubramanian et. al. describes a method for refining an existing printer calibration using a small number of color patch measurements.
U.S. Pat. No. 5,809,213 to Bhattachaiya provides a method and apparatus for automatic color correction in which a nonlinear interpolation technique is applied to a relatively small number of measured sample values from color patches.
U.S. Pat. Ser. No. 09/012,651 to Rumph takes color calibration a step further by describing a method that maintains color fidelity taking into account object type, noting that objects rendered with different halftones and different rendering intents must necessarily be calibrated separately, since the response of a printing system to different halftones drifts at different rates.
These patents describe a number of different techniques for measuring various numbers and arrangements of color patches in order to keep the color printing system stable with respect to color rendition.
However, nowhere in the art is the problem addressed of printer defects being measured in extent and intensity in order to track changes in defect signature and change the pre-compensations applied to digital data to correct the defects with greater stability.
To solve this problem, the invention describes a method and apparatus for measuring printer defects at appropriate intervals of time and using the measurement data to correct pre-compensations applied to digital data. The measuring instruments used and the measurements taken are more complex than those that are used to measure color patches, because the printer defects being measured consist not of simple color patches but of subtle unwanted changes of color intensity in two directions over small spatial, areas.
For example, a xerographic printing problem called trail-edge deletion can occur when a color fill with sufficient size in the process direction is printed. At the lower process direction edge, for electrostatic reasons having to do with the approaching edge, the toner is often gradually depleted, resulting in an increasingly lighter color as the printer approaches the edge. This printing problem may be corrected by applying a function which changes the density of the color near the edge as a function of the scanline distance from the edge, as described in co-pending application "Anamorphic Object Optimized Function Application for Printer Defect Pre-Compensation". Note that the function is anamorphic (since only process direction edges are modified) and therefore the measurement of gradual color deletion as the edge approaches must in this case be made on a process direction edge. Depending on environmental factors such as humidity, on paper type, on machine age and amount of recent use, and on the age and state of the toner, the intensity of deletion, the spatial extent of the deletion, and the rapidity of the dropoff at the edge all can subtly but measurably change. Without measurements at intervals to track these changes over time, the pre-compensation applied to the edge to correct for trail-edge deletion will not be as successful as if the signature of the tail-edge deletion had been measured recently.
Note that not all printing defects change characteristics over time. The resolution of text edges, for example, is fixed by system resolution and choice of halftone. Available color levels for images is also fixed by image halftone choice and does not vary significantly.
However, there is a large class of printer defects which do drift slightly but significantly in both extent and intensity over time. The amount of mottle in a color fill can vary over time with environmental and paper changes. Similarly, trail-edge deletion, starvation, misregistration, haloing, streaking, and other defects have the characteristic of being subtle but visible unwanted variations of color over a small area of the page, and are often sensitive to slow-changing variables such as environment, paper type, machine state, toner and ink age, and so on. As such, they are subject to being controlled with greater stability if careful measurements are taken and the results used to change the pre-compensation functions for those defects.
As will be made clear, the test patterns used for measurement must comprehend the range of object types; a trail-edge deletion for text halftones wrn be significantly different from that for image halftones. They must also take into account the anamorphic mature of printer defects; i.e., they must allow for measurement of edges in both the scan and process directions.